This invention relates to a method of and apparatus for expressing tone image by digital recording.
In transmitting an original document, many facsimile systems divide the picture data on the document into picture elements and then form an analog picture signal indicating the densities of the picture elements. The facsimile systems then binary-encode the analog picture signal using a predetermined threshold level and thereby create a digital picture signal identifying "white" and "black" picture elements of the original document. The facsimile systems contain a device that records picture data according to the picture elements (printing dots) represented in the digital picture signal. Because this method records picture elements according to binary-coded picture signals, it is impossible to express half-tones.
To overcome this limitation, several methods of reproducing picture signal levels using half-tones have been proposed. Examples of such methods are the dither method, the area gradation method, and the 3L method. In each of these methods, several recording picture elements are grouped to form a unit for expressing a half-tone. The units, referred to as "half-tone expressing units," are closely arranged on a recording surface. The half-tone data from those units determines the printing of the recording picture elements.
FIG. 1 shows the simplest half-tone expressing unit containing recording picture elements arranged in a 2.times.2 matrix. As FIG. 1 shows, the different recording picture elements in the units each have different threshold levels 1.sub.1 through 1.sub.4 which are used to determine the elements' binary state, "0" or "1", for a white or black picture element, respectively. For purposes of explaining FIG. 1, threshold level 1.sub.1 represents the lowest density, threshold level 1.sub.2 is higher than threshold level 1.sub.1, threshold level 1.sub.3 is higher than threshold level 1.sub.2, and threshold level 1.sub.4 represents the highest density.
A picture signal for a white region in an original document has the lowest density and the corresponding signal level is lower than threshold levels 1.sub.1 through 1.sub.4. The half-tone expressing unit for that region thus contains four recording picture elements which are white elements, i.e., non-printing dots, as shown in FIG. 2a. As the density increases, the half-tone level picture signal exceeds threshold level 1.sub.1 causing one recording picture element in the half-tone expressing unit to be represented by a black picture element, e.g., a printing dot as shown in FIG. 2b. Similarly, as the density expressed by the half-tone expressing unit increases still further, the picture signal level exceeds threshold levels 1.sub.2, 1.sub.3 and 1.sub.4 as shown in FIGS. 2(c)-(e). A 2.times.2 matrix of recording picture elements can thus express five gradations from white to black.
FIG. 3 is a graph showing the relationship of picture signal levels and the half-tone levels for threshold level 1.sub.2. As shown in FIG. 3, when the picture signal level reaches the threshold level, it changes from "0" to "1."
While a binary-encoding of picture signal levels can express five half-tone gradations, ternary-encoding can express even more gradations.
FIG. 4 shows examples of a half-tone expressing unit formed according to a conventional ternary half-tone expression method. Each of the four recording elements in a half-tone expressing unit has two threshold levels. An element is expressed as a white picture element, referred to as a non-printing dot, when its picture signal level is below the lower of the two threshold levels. An element is expressed as a black picture element, referred to as "a full dot," when its picture signal level is above the higher of the two threshold levels. When the element's picture signal level is between the two threshold levels, it is expressed as a picture element whose black area is smaller than that of the full dot and is referred to as "a half dot."
FIG. 5 shows the variations in half-tone levels with picture signal levels for the recording picture element having threshold level 1.sub.1 in the half-tone expressing unit shown in FIG. 1. Conventional devices are limited by the principle that, as the picture signal density level increases, the half-tone level of each recording picture element must also increase in a stepwise manner such as that shown in FIGS. 4 and 5. These conventional devices can therefore express only nine different gradations as shown in FIGS. 4(a)-(i) and they do not permit use of the six gradations shown in FIGS. 6(a)-(f). Devices employing ternary-encoding and having four recording picture elements in each half-tone expressing unit should be able to express fifteen (15) different gradations, i.e., the nine of FIGS. 4(a)-(i) and the six of FIGS. 6(a)-(f). Conventional devices, however, have not been able to use this full capability.